1. Field of the Invention
This invention relates to a device for testing the characteristics of material surrounding a bore hole, and more particularly to a bore hole test probe utilizing an expansion membrance which is pressurized to contact the walls of the bore hole.
2. Description of the Prior Art
It is important in a variety of fields to determine characteristics of soil, rocks and the like at various levels beneath the surface of the ground. For example, it is important to note the properties of a site on which a building is to be constructed in order to properly design for settling of the building.
One technique for measuring soil properties at various levels is described in British Pat. No. 1,430,239 issued to National Research Development Corporation. The National Research device is commercially available and in common use, and it is known as a "camkometer" probe. Basically, the camkometer is a probe which is inserted into a usually vertical bore hole to determine the properties of the material surrounding the bore hole. The camkometer includes a cylindrical body surrounded by a resilient expansion membrane. The body, which is secured to the lower end of a pipe extending into the bore hole, has a hollow core which communicates with the interior of the pipe. The probe also includes pivotally mounted feeler members contacting the inner surface of the expansion membrane along a line to measure its expansion. In use, pressure is applied to the pipe from above, causing the expansion membrane to expand against the wall of the bore hole. At the same time, the expansion of the membrane is measured by the feeler members, and this expansion in relation to the pressure applied to the pipe provides an indication of the properties of the material surrounding the bore hole.
Although the above-described bore hole test probe is satisfactory for most uses, it is nevertheless limited in both its accuracy and the pressure which may be applied to the expansion membrane. When the expansion membrane contacts the wall of the bore hole, the pressure differential, and hence the shear stress across the membrane, approaches zero since the outwardly directed force of the fluid pressure is equalized by the inwardly directed force of the wall of the bore hole. However, the ends of the expansion membrane extending between the probe and the wall of the bore hole are subject to substantial shear forces, because this area of the expansion membrane does not contact the wall of the bore hole and thus there is; nothing to generate a force opposing the force produced by the fluid pressure. Consequently, the expansion membrane has a tendency to rupture near its junction with the body. To avoid rupture, the pressure applied to the expansion member must be limited to a relatively low value. Yet, some soil conditions require a relatively high pressure to be applied to the test probe.
Another limitation in the accuracy of the abovedescribed bore hole probe stems from a lack of uniformity in the expansion of the membrane resulting from the nonhomogeneity of the material surrounding the bore hole. The feeler members in the conventionally used bore hole test probe contact the inner surface of the expansion membrane along a relatively small area. Yet the expansion of the membrane in the area may differ significantly from the average expansion of the membrane in the vicinity of the feeler members, and it is the average expansion which is most indicative of the properties of the material surrounding the bore hole.
Both of the aforementioned limitations of the conventionally used bore hole test probe (1) reduce the accuracy of measurements, and (2) limit pressures to be applied to the expansion membrane.